A small, nonprotein, water-soluble molecule or ion, such as a calcium ion (Ca) or cyclic AMP, that relays a signal to a cell’s interior in response to a signaling molecule bound by a signal receptor protein.

Cyclic adenosine monophosphate, a ring-shaped molecule made from ATP that is a common intracellular signaling molecule (second messenger) in eukaryotic cells. It is also a regulator of some bacterial operons.